1. Field of the Invention
The present invention relates to an improved cell package which can be welded to hermetically seal the cell package after the cell stack and electrolyte have been placed in the package and a method of manufacturing the same.
2. Background
A important consideration in the manufacturing of electrochemical batteries is the manner in which the electrolyte is introduced into the cell stack. As discussed in a related patent application, one current technique includes the steps of pouring the electrolyte into the cell stack during the manufacturing of the cell stack in a machine, placing the electrolyte impregnated cell stack into the cell package, evacuating the cell package and heat sealing the package.
This technique has shortcomings. One potential concern is the loss of electrolyte during the step of pouring the electrolyte into the cell stack and the subsequent step of evacuating the package. The electrolyte is a relatively expensive component of the electrochemical cell. Thus, the loss of electrolyte increases the overall cost of manufacturing the cell.
A second potential concern is that the electrolyte that is suctioned from the cell stack during the evacuating step contaminates the inside of the package. Such contamination of the package may make it difficult to securely seal the package. As such, subsequent leakage of the electrolyte from the sealed package may result. A further concern is that the pouring step must be performed in a glove box environment (i.e., dry and inert atmosphere). Since this step is an intermediate step in the manufacturing of the cell stack, the machine which manufactures the cell stack must consequently have a glove box environment, thus driving up the cost of the machine. In addition, when the electrolyte is poured into the cell stack, the electrolyte contaminates the machine.
The current cell package is formed of a laminate of a polyester outer layer, an aluminum barrier layer and a polyethylene or polypropylene inner layer. The polyester layer provides strength, the aluminum layer prevents water from penetrating the cell package and the inner layer allows for the heat sealing of the cell package. Specifically, generally, the cell package includes two parts that are bonded together around their periphery by heat sealing the inner layers to each other. The problem with this laminate is that once contaminated with electrolyte, the inner layers may not form a secure heat seal. This makes degassing and resealing of the cell package a problem. Finally, when a polymer is used as an inner layer, the electrolyte may still be able to permeate through the polymer itself. This is especially true if the battery is exposed to elevated temperatures.
It is an object of the invention to provide a method of manufacturing an electrochemical cell which overcomes the above problems. In particular, an object of the invention is to provide a method of manufacturing a cell where the electrolyte is introduced into the cell stack with minimal or no loss of electrolyte. Another object of the invention is to provide a method in which the electrolyte filling step is performed after the cell stack is manufactured so that the cell stack manufacturing machine does not have to maintain a glove box environment and contamination of the machine is eliminated. Another object of the invention is to provide a packaging, which minimizes the possibility of electrolyte permeation through the seals.
These and other objects are achieved by a method of fabricating an electrochemical cell, comprising the steps of forming an electrode cell stack and a metallic cell package having a base portion and a lid portion which are welded to each other to define an enclosure, the cell package including a first section for receiving the cell stack and a second section having an inlet port which communicates with the first section; placing the cell stack into the enclosure in the first section; sealing the lid portion to the base portion around a periphery of the cell package to form a peripheral seam; applying a vacuum to the enclosure through the inlet port in the cell package; introducing an electrolyte into the enclosure via the inlet port; and welding the lid portion to the base portion to form a first weld seam located between the first and second sections to seal off the first section from the second section. The first weld seam extends from the peripheral seam on a first side of the cell package to the peripheral seam a second side of the cell package.
The method further includes the steps of partially charging the cell stack resulting in generation of gases inside the enclosure; puncturing the cell package to form an evacuation port located in a third section of the cell; applying a vacuum to the evacuation port of the cell stack to withdraw the gases; and sealing the lid portion to the base portion across a second weld seam located between the first section and the third section. According to one preferred aspect of the invention the second weld seam extends from the first side to a third side opposite the first side. After the second weld seam has been formed, the excess portions of the cell package, corresponding to the second and third sections, are removed from the first section which holds the cell stack.
The electrochemical cell according to a preferred embodiment of the invention comprises: a casing; and an electrode cell stack contained within the casing along with an electrolyte. The casing includes a base and a lid that are made of a metallic material such that they can be welded to each other along a seam weld to form an enclosure for receiving the cell stack. This is different from conventional cell packages where the parts are heat sealed to each other. The metallic material of the casing is, for example, aluminum, copper, nickel or stainless steel. The weld seam extends around a periphery of the cell package.
The cell stack includes first and second tabs of opposite polarity. The electrochemical cell further comprises a pass-through terminal secured to the casing and electrically connected to the second tab while the first tab is electrically connected to the casing. According to one aspect of the invention, the pass-through terminal comprises an eyelet having a first through-hole, an insulator located in the first through-hole of the eyelet and having a second through-hole; and a terminal post located in the second through-hole of the insulator so as to be insulated from the eyelet, wherein the eyelet is welded to the metallic material of the casing and the terminal post is electrically connected to the second tab of the cell stack. If the casing is made of copper, the eyelet is nickel plated iron, the insulator is glass and the terminal post is molybdenum. Also, the terminal post and the second tab have a positive polarity and the first terminal and the copper casing have a negative polarity.
On the other hand, when the casing is made of aluminum, the eyelet is aluminum, the insulator is ceramic and the terminal post is copper. In this case the terminal post and the second tab have a negative polarity and the first terminal and the aluminum casing have a positive polarity.
According to another aspect of the invention, when the casing is aluminum, the pass-through terminal comprises a copper rivet, at least one insulator circumscribing the rivet so as to insulate the rivet from the casing, and a nickel washer disposed on an outside of the casing and contacting the rivet with the insulator insulating the washer from the casing. In this case, the rivet and the second tab have a negative polarity and the first terminal and the aluminum casing have a positive polarity.
With the above electrochemical cell and related fabrication technique there is little or no electrolyte loss. In particular, since the electrolyte is injected into the electrode cell stack after the package has been sealed, substantially all of the electrolyte is suctioned into the electrode cell stack without any of the electrolyte escaping from the package. In addition, contamination of the cell manufacturing machine with electrolyte is minimized. Accordingly, all of the concerns discussed above with respect to the current technique are overcome.
Further, since the casing is made of a metallic material such as copper or aluminum, the package can be sealed by welding, instead of by heating. It has been discovered that electrolyte contamination does not interfere with a welded seal. The welding process can be, but is not limited to, ultrasonic welding.